The present invention relates generally to noise reduction, and in particular to a noise reduction circuit which eliminates impulse noise that occurs in audio signals using a linear interpolation technique immune to white noise. The invention is particularly useful for applications where the audio signal level is relatively low in comparison with the white noise level.
One method currently available for suppressing impulse noise that contaminates audio signals involves reducing the transmission gain or shutting off the transmission path of the signal as long as the noise is present. Another method involves detecting the amplitude of the wanted signal on the rising edge of an impulse noise and retaining the detected amplitude in the presence of the impulse noise. While these methods are effective in suppressing impulse noise, the noise-affected portion of the signal is not reconstructed, resulting in unnatural sound. To overcome this problem modern digital audio systems utilize linear interpolation technique to predict the original waveform of the noise-affected portion by linear interpolation. This type of systems requires complicated, expensive circuitry, not suitable for moderate cost equipments.
The aforesaid Copending U.S. Application discloses an impulse noise reduction circuit in which the audio signal is passed through a first sample-and-hold circuit which tracks the waveform of the signal when no impulse noise is present and holds the signal level in response to the impulse noise. The slope ratio of the audio signal is detected by a differentiator and sampled by a second sample-and-hold circuit in response to that impulse noise as an indication of the position of the noise of interest in the audio signal waveform. The sampled signal drives a voltage-controlled bidirectional constant current source to linearly vary the voltage sampled by the first sample-and-hold by performing linear charging and discharging of the capacitor thereof.
However, if white noise prevails in the high frequency range of the audio spectrum and the wanted signal level is low relative to the white noise, the white noise is detected by the differentiator and causes an increase in the level of the slope ratio signal which is to be sampled during the noise suppression period. This results in the interpolating voltage deviating from what it should be. Another disadvantage is that an additional noise is introduced by the randomness of the white noise.